DESCRIPTION: Cancer development involves alterations in both cellular protooncogenes and in tumor suppressor genes. In vitro systems have provided molecular details of the pathways potentially disrupted in this multi-step process. However, understanding tumorigenesis of diverse cell types clearly requires the use of in vivo systems such as transgenic and "knock-out" mouse models. The applicant has established transgenic tumor models based on the fact that tumor virus oncoproteins such as SV40 T antigen (T-Ag) inactivate key cellular growth regulatory proteins, including p53 and pRb. Using tissue-specific promoters to express wild-type and mutant T-Ags, we have explored the function of these tumor suppressors within the animal. Using transgenic and knock-out animals we recently showed that, in brain epithelial cells, p53-dependent apoptosis is induced in response to aberrant proliferation resulting in part from inactivation of the Rb family proteins. Loss of p53 function results in tumor progression coincident with a reduction in apoptosis. Moreover, inactivation of p53 is selected for as tumors develop in a p53 +/-background. These results suggest a mechanism by which p53 loss contributes to tumor progression in human cancer. Her continuation of these studies proposed herein includes the following aims: (1) To delineate key regulatory components of the p53-dependent apoptosis pathway. Directed studies involving crosses or chimera analyses with existing knockout strains will determine whether known or implicated factors are involved (e.g., p21, bax, mdm2, gadd45, E2F1). Further, genetic analyses of tumors arising in the p53 +/-mice will be used to determine whether those that have retained functional p53 have inactivated other components of the pathway, novel or known. (2) To determine the contribution of genetic instability to tumor progression, comparative genome analysis and simple sequence length polymorphism analysis of p53 _ and p53 - tumors will be performed to determine the extent and consistency of genetic aberration. (3) To determine whether p53 dependent apoptosis has a similar role in other cell types which display a high frequency of p53 mutation of human cancer. Here, studies are proposed for mammary epithelial cells. For further cell-specific studies, tissue specific inactivation of p53 by somatic delivery of a dominant negative mutant of p53 will be tested.